Each incandescant lightbulb in a home draws between 40 and 150 watts of power when turned on. Most bulbs used draw 60 watts. If you had 3 lights on for 6 hours of the day, that would be over 1 killowatt-hour.
If each of those bulbs was replaced with a 13 watt compact fluorescant, it would be just over 2/10 ths of a killowatt-hour.
Compact fluorescant (CFL) bulbs fit in ordinary sockets, come in several colors (I prefer the yellower ones), and are far more efficient than incandescant bulbs. Some power companies are offering incentives for switching to them.
The better insulated a home is, the less air conditioning it needs to use. If you own your own home, you can improve its insulation by:
The best way to improve your power efficiency is to turn off the monitor completely when not using it.
The wattage listed on a computer power supply is its maximum. What it actually uses will depend on the system load attached to it. In general, though, laptop computers tend to be more efficient than desktops, and LCD monitors tend to be more efficient than CRT ones.
Your television might be drawing "phantom power" when it is turned off. The only way I know of to be sure is to plug it into a watt meter.
The central air will be more efficient if its filters are kept clean. When the filters are dirty the motor can push less air through them, which makes it less effective.
Washable filters provide the advantages of a filter that isn't clogged, without upkeep expense every 1-3 months.